The invention relates to techniques for characterizing the accuracy of genome sequence analysis, and more particularly to the use of advanced mathematical methods for correction of observational errors in sequences having portions of known content.
Large-scale genomic sequence analysis (“sequencing”) is a key step toward understanding a wide range of biological phenomena. The need for low-cost, high-throughput sequencing and re-sequencing has led to the development of new approaches to sequencing that employ parallel analysis of multiple nucleic acid targets simultaneously.
Conventional methods of sequencing are generally restricted to determining a few tens of nucleotides before signals become significantly degraded, thus placing a significant limit on overall sequencing efficiency. Conventional methods of sequencing are also often limited by signal-to-noise ratios that render such methods unsuitable for single-molecule sequencing.
A challenge of genome sequencing is the accurate recognition, identification, characterization and classification of DNA strands. Efforts have been developed for improving DNA sensing, analysis and measurement throughput by manipulation of DNA including the manipulation of DNA nanoballs (“DNB”). The techniques for sequencing DNB involve the categorization of fluorophore responses of DNB at genome attachment sites on rigid substrates in the presence of interference from adjacent attachment sites. A specific categorization is known as a call, as hereinafter explained. Signal-to-noise ratios in DNA sequencing can be relatively low, which adversely impacts base quality score.
Improvements in base quality score would allow better characterization of the sequencing system and its failure modes. Improvements would also allow one to quantify improvements in such aspects as the substrate, the biochemistry, the methodology of preparation of samples, the mechanical systems and optical systems, and the mathematical algorithms that analyze and yield the calls.
Linear block cyclic symbol-based error correction methods relying on error correcting codes have been used to identify and correct bit streams in impaired communication channels, subject to limitations on error rate and run length. Types of codes used in the past for error correction of bit errors in DNA are the Hamming codes. These codes are capable of correcting for one bit error but not one base error. However, Hamming codes are not capable of correcting a large number of errors in a sequence.
Reed-Solomon error detection/correction is a method based on an error-correcting code that works by oversampling a polynomial constructed from the data. Sampling the polynomial more often than is necessary makes the polynomial over-determined. As long as more than a minimum number of the samples are correct, the original polynomial can be recovered in the presence of a some bad points. The relationship between to good and bad points determines the number of errors that can be corrected.
Reed-Solomon codes have been explained at length in the mathematics and communication literature. See for example Error Control Coding: Fundamentals and Applications by Shu Lin and Daniel Costello; Prentice Hall; and Error Control Systems for Digital Communication and Storage, by Stephen B. Wicker; Prentice Hall. It has been shown that if it is guaranteed that there are less than one error in a string of seven values in a sequence having four possible values, then the related mathematics can guarantee that an error in the seven-member long sequence can be captured and corrected.
It would be advantageous for the field of genome analysis if methods could be designed to characterize and potentially increase the accuracy and call-rate/efficiency of sequencing.
In the following description, numerous specific details are set forth to provide a more thorough understanding of the present invention. However, it will be apparent to one of skill in the art that the present invention may be practiced without one or more of these specific details. In other instances, well-known features and procedures well known to those skilled in the art have not been described in order to avoid obscuring the invention.
The practice of genome analysis may employ, unless otherwise indicated, conventional techniques and descriptions of organic chemistry, polymer technology, molecular biology (including recombinant techniques), cell biology, biochemistry, and immunology, which are within the skill of the art. Such conventional techniques include polymer array synthesis, hybridization, ligation, and detection of hybridization using a label. The present invention focuses on the detection problem. Conventional techniques and descriptions can be found in standard laboratory manuals such as Genome Analysis: A Laboratory Manual Series (Vols. I-IV), Using Antibodies: A Laboratory Manual, Cells: A Laboratory Manual, PCR Primer: A Laboratory Manual, and Molecular Cloning: A Laboratory Manual (all from Cold Spring Harbor Laboratory Press); Stryer, L. (1995) Biochemistry (4th Ed.) Freeman, New York, Gait, “Oligonucleotide Synthesis: A Practical Approach,” 1984, IRL Press, London, Nelson and Cox (2000); Lehninger, Principles of Biochemistry 3rd Ed., W. H. Freeman Pub., New York, N.Y. and Berg et al. (2002) Biochemistry, 5th Ed., W. H. Freeman Pub., New York, N.Y., all of which are herein incorporated in their entirety by reference for all purposes.
As used herein and in the appended claims, the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to “a polymerase” refers to one agent or mixtures of such agents, and reference to “the method” includes reference to equivalent steps and methods known to those skilled in the art, and so forth.
Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. All publications mentioned herein are incorporated herein by reference for the purpose of describing and disclosing devices, compositions, formulations and methodologies which are described in the publication and which might be used in connection with the presently described invention.
Where a range of values is provided, it is understood that each intervening value, to the tenth of the unit of the lower limit unless the context clearly dictates otherwise, between the upper and lower limit of that range and any other stated or intervening value in that stated range is encompassed within the invention. The upper and lower limits of these smaller ranges may independently be included in the smaller ranges is also encompassed within the invention, subject to any specifically excluded limit in the stated range. Where the stated range includes one or both of the limits, ranges excluding either both of those included limits are also included in the invention.
The following definitions may be helpful in providing background for an understanding of the invention.
“Adaptor” refers to an engineered construct comprising “adaptor elements” where one or more adaptors may be interspersed within target nucleic acid in a library construct. The adaptor elements or features included in any adaptor vary widely depending on the use of the adaptors, but typically include sites for restriction endonuclease recognition and/or cutting, sites for primer binding (for amplifying the library constructs) or anchor primer binding (for sequencing the target nucleic acids in the library constructs), nickase sites, and the like. In some aspects, adaptors are engineered so as to comprise one or more of the following: 1) a length of about 20 to about 250 nucleotides, or about 40 to about 100 oligonucleotides, or less than about 60 nucleotides, or less than about 50 nucleotides; 2) features so as to be ligated to the target nucleic acid as at least one and typically two “arms”; 3) different and distinct anchor binding sites at the 5′ and/or the 3′ ends of the adaptor for use in sequencing of adjacent target nucleic acid; and 4) optionally one or more restriction sites
“Amplicon” means the product of a polynucleotide amplification reaction. That is, it is a population of polynucleotides that are replicated from one or more starting sequences. Amplicons may be produced by a variety of amplification reactions, including but not limited to polymerase chain reactions (PCRs), linear polymerase reactions, nucleic acid sequence-based amplification, circle dependant amplification and like reactions (see, e.g., U.S. Pat. Nos. 4,683,195; 4,965,188; 4,683,202; 4,800159; 5,210,015; 6,174,670; 5,399,491; 6,287,824 and 5,854,033; and US Pub. No. 2006/0024711).
“Circle dependant replication” or “CDR” refers to multiple displacement amplification of a double-stranded circular template using one or more primers annealing to the same strand of the circular template to generate products representing only one strand of the template. In CDR, no additional primer binding sites are generated and the amount of product increases only linearly with time. The primer(s) used may be of a random sequence (e.g., one or more random hexamers) or may have a specific sequence to select for amplification of a desired product. Without further modification of the end product, CDR often results in the creation of a linear construct having multiple copies of a strand of the circular template in tandem, i.e. a linear, concatamer of multiple copies of a strand of the template.
“Circle dependant amplification” or “CDA” refers to multiple displacement amplification of a double-stranded circular template using primers annealing to both strands of the circular template to generate products representing both strands of the template, resulting in a cascade of multiple-hybridization, primer-extension and strand-displacement events. This leads to an exponential increase in the number of primer binding sites, with a consequent exponential increase in the amount of product generated over time. The primers used may be of a random sequence (e.g., random hexamers) or may have a specific sequence to select for amplification of a desired product. CDA results in a set of concatameric double-stranded fragments.
“Complementary” or “substantially complementary” refers to the hybridization or base pairing or the formation of a duplex between nucleotides or nucleic acids, such as, for instance, between the two strands of a double-stranded DNA molecule or between an oligonucleotide primer and a primer binding site on a single-stranded nucleic acid. Complementary nucleotides are, generally, A and T (or A and U), or C and G. (Univeral bases may be used in some appropriate in some applications.) Two single-stranded RNA or DNA molecules are said to be substantially complementary when the nucleotides of one strand, optimally aligned and compared and with appropriate nucleotide insertions or deletions, pair with at least about 80% of the other strand, usually at least about 90% to about 95%, and even about 98% to about 100%.
“Duplex” means at least two oligonucleotides or polynucleotides that are fully or partially complementary and which undergo Watson-Crick type base pairing among all or most of their nucleotides so that a stable complex is formed. The terms “annealing” and “hybridization” are used interchangeably to mean formation of a stable duplex. “Perfectly matched” in reference to a duplex means that the poly- or oligonucleotide strands making up the duplex form a double-stranded structure with one another such that every nucleotide in each strand undergoes Watson-Crick base pairing with a nucleotide in the other strand. A “mismatch” in a duplex between two oligonucleotides or polynucleotides means that a pair of nucleotides in the duplex fails to undergo Watson-Crick base pairing.
“Hybridization” refers to the process in which two single-stranded polynucleotides bind non-covalently to form a stable double-stranded polynucleotide. The resulting (usually) double-stranded polynucleotide is a “hybrid” or “duplex.” “Hybridization conditions” will typically include salt concentrations of less than about 1M, more usually less than about 500 mM and may be less than about 200 mM. A “hybridization buffer” is a buffered salt solution such as 5% SSPE, or other such buffers known in the art. Hybridization temperatures can be as low as 5° C., but are typically greater than 22° C., and more typically greater than about 30° C., and typically in excess of 37° C. Hybridizations are usually performed under stringent conditions, i.e., conditions under which a probe will hybridize to its target subsequence but will not hybridize to the other, uncomplimentary sequences. Stringent conditions are sequence-dependent and are different in different circumstances. For example, longer fragments may require higher hybridization temperatures for specific hybridization than short fragments. As other factors may affect the stringency of hybridization, including base composition and length of the complementary strands, presence of organic solvents, and the extent of base mismatching, the combination of parameters is more important than the absolute measure of any one parameter alone. Generally stringent conditions are selected to be about 5° C. lower than the Tm for the specific sequence at a defined ionic strength and pH. Exemplary stringent conditions include a salt concentration of at least 0.01M to no more than 1M sodium ion concentration (or other salt) at a pH of about 7.0 to about 8.3 and a temperature of at least 25° C. For example, conditions of 5× SSPE (750 mM NaCl, 50 mM sodium phosphate, 5 mM EDTA at pH 7.4) and a temperature of 30° C. are suitable for allele-specific probe hybridizations.
“Isolated” means substantially separated or purified away from contaminants by standard methods. In the case of biological heteropolymers such as polynucleotides (DNA, RNA, etc.) for example, the polynucleotide is substantially separated or purified away from other polynucleotides and other contaminants that are present in the cell of the organism in which the polynucleotide naturally occurs. The term “isolated” also means chemically synthesized or, in the case of a polynucleotide or polypeptide, produced by recombinant expression in a host cell.
“Ligation” means to form a covalent bond or linkage between the termini of two or more nucleic acids, e.g., oligonucleotides and/or polynucleotides, in a template-driven reaction. The nature of the bond or linkage may vary widely and the ligation may be carried out enzymatically or chemically. As used herein, ligations are usually carried out enzymatically to form a phosphodiester linkage between a 5′ carbon terminal nucleotide of one oligonucleotide with a 3′ carbon of another nucleotide. Template driven ligation reactions are described in the following references: U.S. Pat. Nos. 4,883,750; 5,476,930; 5,593,826; and 5,871,921.
“Microarray” or “array” refers to a solid phase support having a surface, preferably but not exclusively a planar or substantially planar surface, which carries an array of sites containing nucleic acids such that each site of the array comprises identical copies of oligonucleotides or polynucleotides and is spatially defined and not substantially overlapping with other member sites of the array; that is, the sites are spatially discrete. The array or microarray can also comprise a non-planar structure with a surface such as a bead or a well. The oligonucleotides or polynucleotides of the array may be covalently bound to the solid support, or may be non-covalently bound. Conventional microarray technology is reviewed in, e.g., Schena, Ed. (2000), Microarrays: A Practical Approach (IRL Press, Oxford). As used herein, “random array” or “random microarray” refers to a microarray where the identity of the oligonucleotides or polynucleotides is not discernable, at least initially, from their location but may be determined by a particular operation on the array, such as by sequencing, hybridizing decoding probes or the like. See, e.g., U.S. Pat. Nos. 6,396,995; 6,544,732; 6,401,267; and 7,070,927; WO publications WO 2006/073504 and 2005/082098; and US Pub Nos. 2007/0207482 and 2007/0087362.
“Nucleic acid”, “oligonucleotide”, “polynucleotide”, “oligo” or grammatical equivalents used herein refers generally to at least two nucleotides covalently linked together. A nucleic acid generally will contain phosphodiester bonds, although in some cases nucleic acid analogs may be included that have alternative backbones such as phosphoramidite, phosphorodithioate, or methylphophoroamidite linkages; or peptide nucleic acid backbones and linkages. Other analog nucleic acids include those with bicyclic structures including locked nucleic acids, positive backbones, non-ionic backbones and non-ribose backbones. Modifications of the ribose-phosphate backbone may be done to increase the stability of the molecules; for example, PNA:DNA hybrids can exhibit higher stability in some environments.
“Preselected,” when used in reference to a block of a monomer subunit sequence that has a coding corresponding to at least one Reed-Solomon code, means that the heteropolymer is designed and/or synthesized to include the block of monomer sequence or that the block of monomer sequence is added to a preexisting heteropolymer sequence. For example, by way of illustration, a polynucleotide can be designed to include a block of five to ten nucleotide bases, such as the seven or ten nucleotide base sequences described herein, or a polynucleotide containing such a five to ten base block can be added to a preexisting polynucleotide by ligation or other standard methods.
“Primer” means an oligonucleotide, either natural or synthetic, which is capable, upon forming a duplex with a polynucleotide template, of acting as a point of initiation of nucleic acid synthesis and being extended from its 3′ end along the template so that an extended duplex is formed. The sequence of nucleotides added during the extension process is determined by the sequence of the template polynucleotide. Primers usually are extended by a DNA polymerase.
“Probe” means generally an oligonucleotide that is complementary to an oligonucleotide or target nucleic acid under investigation. Probes used in certain aspects of the claimed invention are labeled in a way that permits detection, e.g., with a fluorescent or other optically-discernable tag.
“Sequence determination” in reference to a target nucleic acid means determination of information relating to the sequence of nucleotides in the target nucleic acid. Such information may include the identification or determination of partial as well as full sequence information of the target nucleic acid. The sequence information may be determined with varying degrees of statistical reliability or confidence. In one aspect, the term includes the determination of the identity and ordering of a plurality of contiguous nucleotides in a target nucleic acid starting from different nucleotides in the target nucleic acid.
“Substrate” refers to a solid phase support having a surface, usually planar or substantially planar, which carries an array of sites for attachment of nucleic acid macromolecules such that each site of the array is spatially defined and not overlapping with other member sites of the array; that is, the sites are spatially discrete and optically resolvable. The nucleic acid macromolecules of the substrates of the invention may be covalently bound to the solid support, or may be non-covalently bound, i.e. through electrostatic forces. Conventional microarray technology is reviewed in, e.g., Schena, Ed. (2000), Microarrays: A Practical Approach (IRL Press, Oxford).
“Macromolecule” used in relation to a nucleic acid means a nucleic acid having a measurable three dimensional structure, including linear nucleic acid molecules with comprising secondary structures (e.g., amplicons), branched nucleic acid molecules, and multiple separate copies of individual with interacting structural elements, e.g., complementary sequences, palindromes, or other sequence inserts that cause three-dimensional structural elements in the nucleic acid.
“Target nucleic acid” means a nucleic acid from a gene, a regulatory element, genomic DNA, cDNA, RNAs including mRNAs, rRNAs, siRNAs, miRNAs and the like and fragments thereof. A target nucleic acid may be a nucleic acid from a sample, or a secondary nucleic acid such as a product of an amplification reaction.
Although the present invention is described primarily with reference to specific embodiments, it is also envisioned that other embodiments will become apparent to those skilled in the art upon reading the present disclosure, and it is intended that such embodiments be contained within the present inventive methods.